Wells have long been drilled using drilling mud as the circulating medium. The mud performs two functions: it provides a column of heavy fluid that exerts hydrostatic pressure at the bottom of the wellbore, to prevent entry into the wellbore of pressurized hydrocarbons present in the formation being drilled; and it serves to carry solid cuttings up and out of the wellbore.
When drilling mud is used, there is a likelihood that it will penetrate out into a porous and permeable, hydrocarbon-containing reservoir when the reservoir is being opened up or drilled through. When this occurs, the productivity of the well can be adversely affected. The mud that has penetrated radially into the formation acts to impede the flow of hydrocarbons into the wellbore.
Under-balanced drilling was recently developed as a technique for opening up the pay zone of the reservoir. Typically, the well is completed with casing to the top of the pay zone. Then drilling is initiated into the pay zone using a relatively light circulating fluid, such as water or diesel fuel. The fluid is selected so that the hydrostatic head created by it is less than the expected reservoir pressure. Therefore, the drilling fluid will not invade the reservoir as the drilling string penetrates through it.
However, when this is done there is a likelihood that pressurized hydrocarbons (oil or natural gas) present in the reservoir will be produced into the wellbore. Thus the circulated fluid returning out of the wellbore will often comprise oil, water, gas and suspended solids. In addition, this stream will be under pressure and that are adapted to contain the components of the stream while they are separated and recovered as discrete and separate streams.
By way of contrast, a rig operating with drilling mud normally has an "open" tank system for treating a returning mud stream. More particularly, the mixture of mud containing suspended solid cuttings passes through a vibrating screen assembly (known as a "shale shaker") to separate out the bulk of the cuttings, which drop from the shaker onto the ground or into a pit. The mud then enters one or more rectangular, open-topped "mud tanks". The mud slowly moves through the mud tank and most of the fine solids which remain suspended after screening by the shale shaker settle out. Often the mud tank has one or more transverse weirs or baffles, which divide the tank chamber into compartments. The weir functions to trap settling fine solids and thick mud, allowing "cleaned" mud to advance. The cleaned mud then normally is recycled to the wellbore.
From time to time, the rig crew stop the mud flow, drain the tanks into a pit and flush out settled solids using high pressure water jets or hoses.
It is clear therefore that in conventional mud handling systems it is known:
to provide retention time in the tanks to enable the fine solids to settle out; and PA1 to use weirs in the path of the flow, to trap solids and heavily contaminated mud, while cleaner mud passes over the weir and moves toward the tank outlet. PA1 a bottom outlet for solids removal; PA1 an overhead outlet for gas removal; PA1 outlets at different elevations for separate water and oil removal; PA1 a sparger ring close to the bottom of the vessel chamber so that water could be injected to fluidize and help remove solids out the bottom outlet; and PA1 a feed inlet structure having a T configuration, with its outlet being downwardly directed. PA1 that the volume of the tank was relatively small and the volume of cuttings large. This resulted in having to interrupt drilling periodically to clean the solids from the tank by hooking up a vacuum truck to the bottom outlet to suction out the solids; PA1 that the downwardly directed feed inlet was causing the gassy incoming feed stream to jet down into the liquid and settling solids, thereby stirring them up; and PA1 that the retention time was inadequate, which resulted in the liquid product(s) being contaminated with solids. PA1 A horizontal, elongated, cylindrical pressure vessel having 550 barrels capacity; PA1 An inlet structure that was a tubular involute having its outlet directed horizontally, toward the curved head or end wall of the vessel. The inlet structure was located at about the longitudinal midpoint of the upper section of the vessel chamber. The inlet structure functioned to slow the flow velocity of the incoming feed stream and to introduce it horizontally into the chamber; PA1 A transverse weir extended across the chamber immediately downstream of the inlet structure, dividing the chamber into upstream and downstream compartments. The upstream compartment had sufficient volume to contain all of the coarse solids which the well was expected to produce; PA1 The length of the downstream compartment was sufficient to provide enough retention time to allow the fines (clay and the like) to settle out; PA1 The outlet of the involute inlet structure was spaced above the weir's rim, so that gas could break out and not be entrained in the accumulated liquid; PA1 Oil and water outlet risers were provided downstream of the weir, an overhead gas outlet was provided and solids outlets were provided in the base of the upstream and downstream compartments. PA1 Forming the upstream compartment with a first weir which had its base close to the vessel head (about 30 inches from it) and which slanted upwardly and away from the head, so that the weir, curved head and vessel curved side wall in effect formed a hopper for funnelling settling solids to a solids outlet formed in the vessel side wall at the foot of the compartment; PA1 Providing a solids auger pump connected with the solids outlet of the upstream compartment for removing solids while flow through the vessel continued; PA1 Positioning the involute feed inlet structure over the upstream compartment, to direct the feed stream generally horizontally and toward the adjacent head; PA1 Providing water sparger inlets in the compartment, to fiuidize the solids, if required; PA1 Providing a second weir at about the longitudinal midpoint of the vessel chamber, for trapping settling fines and defining intermediate and downstream compartments; PA1 Providing a solids outlet in the base of the intermediate compartment as well as a tubular riser having an elevated outlet for removing water; PA1 Providing an outlet from the downstream compartment, for removing oil; and PA1 Providing manways leading into each compartment.
These known concepts are incorporated as part of the present invention.